The present invention generally pertains to frequency tuning in a receiver of an RF communications system and is particularly directed to refined tuning of an RF receiver using frequency-locked loop (FLL) tuning.
FLL tuning is increasingly being used for frequency synthesis/tuning. TV converters, for instance, use frequency-locked loops for converting the RF frequencies of received television signals to the frequency of Channel 3 or 4. FLL tuning includes the steps of measuring the frequency of a voltage-controlled oscillator (VCO) output signal, comparing it with a predetermined reference frequency value, and varying the control voltage of the VCO to correct the VCO output signal frequency.
In the RF converter of a typical RF receiver using FLL, a first VCO output signal at a frequency f.sub.1 is mixed with a received input signal at a frequency f.sub.IN to provide the input signal at an intermediate frequency f.sub.IF. The input signal at the intermediate frequency f.sub.IN subsequently is processed by a phase-locked loop (PLL) demodulator, in which the input signal at the intermediate frequency f.sub.IF is mixed with the output signal from a second VCO at a frequency f.sub.2 to provide an input signal at a baseband frequency f.sub.BB. The second VCO tracks the phase of the input signal at the intermediate frequency f.sub.IF and is controlled by a phase error signal e.sub.p derived from the input signal at the baseband frequency f.sub.BB. The frequency f.sub.1 of the first VCO output signal is maintained by a frequency-locked loop in which the frequency f.sub.1 of the first VCO output signal is compared with a predetermined nominal frequency f.sub.n that is the sum or difference of a selected input carrier frequency f.sub.IN and a predetermined intermediate frequency f.sub.IF. An error signal e.sub.1 provided as a result of such comparison is used to control the frequency f.sub.1 of the first VCO output signal.
If there is any frequency drift in the input signal received by the RF converter, the intermediate frequency of the input signal provided to the phase-locked loop demodulator is offset by such frequency drift. Since the output signal from the second VCO is phase locked to the input signal provided to the phase-locked loop demodulator, the frequency of the output signal from the second VCO is also offset by such frequency drift.